(a) Field of Invention
The present disclosure of invention relates to organic light emitting display devices (OLEDD's) and a driving method thereof, and particularly to an organic light emitting display device including a white light emitting pixel in addition to primary color pixels in each picture dot and a driving method for such a four color system.
(b) Description of Related Technology
In recent years, as a substitute for cathode ray tubes (CRTs), flat panel displays have been actively developed. The typical flat panel display includes a plurality of pixels arranged in a rectangular matrix shape and emitting just three primary colors (e.g., R, G, B). A desired one color may be provided by combining primary color outputs emitted from adjacent pixels, and the flat panel display can then display desired images by appropriately controlling the luminance of each primary color pixel.
While most flat panels generally display color images by combining three primary colors such as of red, green, and blue, particularly in the case of organic light emitting devices (OLED's), color images are displayed according to corresponding input image signals by sometimes adding a white light emitting pixel (or a non-color-filtered transparent pixel) in addition to the three primary color emitting pixels to thereby increase luminance. This configuration is called a four-color flat organic light emitting device (4c-OLED). Luminence is increased because there is only so much of an intensity in a 3c-OLED that each of the R, G, B emitters can be safely driven to. So, for example, if each R, G, B emitter has a respective relative drive range between 0 and 255, the maximum luminance that can be output as light corresponds to a drive intensity of FF,FF,FF in terms of hexadecimal expression. However, if a fourth, white light emitting emitter is added, then the maximum luminance that can be output as light extends beyond that associated with the 3c-OLED FF,FF,FF output.
In the four-color flat organic light emitting device (4c-OLED), the three-color image signals that are received as conventional input are often converted into corresponding four-color image signals in order to display images of enhanced or extended luminance on the 4c-OLED. When converting the three-color image signals into four-color image signals, the color impression of three input image signals may be undesirably changed because the added white-light emitter changes the perception of color that would be otherwise seen by the human eye if only the three primary pixels were driven. In terms of greater detail about this, when a first emitter displays a pure color such as yellow (red plus green) and then a supplementing white pixel is added for the purpose of increasing luminance, the eye-perceived color impression of the originally intended, red plus green equals yellow (R+G=Y) is changed. That is, the viewer no longer perceives a pure yellow. Instead the eye may perceive white mixed with yellow, or an off-yellow coloration. Accordingly a distortion phenomenon is added to the intended color impression whereby the color perceived is different than the original yellow color that was intended. To solve this problem, when converting from the three-color image domain into a four-colors emitters domain, the maximum supplement to luminance that is provided by the additional white pixel may have to be limited. However, if the latter is done across a full image of all color permuations, the overall luminance is disadvantageously decreased even when displaying images that do not call for only pure colors such as pure yellow (R+G=Y). This failure to provide maximum luminance is a serious problem because the primary purpose of including a white pixel in the four-color organic light emitting display device is so that luminance can be enhanced.
It is to be understood in reading the above that an organic light emitting diode (OLED) that is the light-emitting element of the organic light emitting display device emits according to a magnitude of current flowing through the organic light emitting diode. Maximum luminance per OLED is often limited because the operational lifetime of the organic light emitting diode tends to decrease according to the magnitude of the current flowing in the organic light emitting diode, and the organic light emitting diode can be damaged (e.g., burned out) if driven with a large current exceeding a predetermined value for too long of a time.
The above information disclosed in this Background section is only for enhancement of understanding of the technologies involved with the present disclosure of invention and therefore it may contain information that does not constitute prior art of a kind that is already publicly known persons of ordinary skill in the art.